Yes, the core is now opensource. When the core is done we will release it to the public with full documentation to ease development of new theme editors.
Any work done on the core will belong to X Themination until its finished. By "finished" we mean all the required features completed.
If you would like to contribute email mstoton@mac.com and we will give you the core and a "task", such as adding support for certain resources. Until we bring up the development forums you'll have to work alone or hook up with the other developers. We will also post the names of everybody who helped on the site and what they did. I would also ask that you know how to handle resources like clr# and clut, icns and PPAT.
Remember, this will be free to the public when finished.
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Project: ThemeShop
Site:
http://xthemination.maccustomise.com